As an instance of the rv_discrete class, planck object inherits from it
a collection of generic methods (see below for the full list),
and completes them with details specific for this particular distribution.

The probability mass function above is defined in the “standardized” form.
To shift distribution use the loc parameter.
Specifically, planck.pmf(k,lambda_,loc) is identically
equivalent to planck.pmf(k-loc,lambda_).